1. Field of the Invention
The present invention relates to a method for manufacturing SIMOX (Separation by Implanted Oxygen) wafers and also to a SIMOX wafer. In particular, the present invention relates to a method for manufacturing SIMOX wafers by a modified low dose (MLD) process.
This application claims priority from Japanese Patent Application No. 2005-172715 filed on Jun. 13, 2005, the content of which is incorporated herein by reference.
2. Background Art
The SIMOX process is known as one of methods for manufacturing silicon-on-insulator (SOI) wafers. In accordance with the process, a buried oxide (BOX) layer can be formed by the following: oxygen atoms are implanted into a wafer, for example, at an implantation energy of about 200 keV and at a dose amount of about 2×1018 atoms/cm2, and then a heat treatment is conducted at a high temperature. In this SIMOX process, since the dose amount and the implantation energy can be closely controlled, the BOX layer and a SOI layer can be uniformly formed at predetermined thicknesses.
Substrates obtained at a dose amount of at least 1018 atoms/cm2 are called “high-dose SIMOX wafers,” and substrates obtained at a dose amount of 4×1017/cm2 or less are called “low-dose SIMOX wafers.” Compared to the case of manufacturing the high-dose SIMOX wafers, in the case of manufacturing the low-dose SIMOX wafers, there is less occurrence of threading dislocations and the amount of time for implanting oxygen ions can be shortened; thereby, high-quality and low-cost SOI substrates can be manufactured. However, as the dose amount is reduced, the thickness of the BOX layer becomes thinner. Therefore, there is a risk of lowering the reliability of the BOX layer.
In order to solve this problem, the ITOX (Internal Thermal Oxidation) process was developed (Patent Document 1 and Non-Patent Document 1). In accordance with this ITOX process, the thickness of the BOX layer can be increased by the following: a heat treatment by which the thickness becomes a theoretical film thickness calculated from the dose amount of oxygen ions, is conducted in an argon gas atmosphere having an oxygen concentration less than 1%, and then a heat treatment is conducted in an argon atmosphere having an oxygen concentration of 1% or more.
By adopting the ITOX process, it is possible to increase the thickness of the BOX layer, to reduce pinholes in the BOX layer, and to lower irregularities or roughness at the surface of the SOI layer (a silicon single crystal layer on the surface of a substrate) and at the interface between the SOI layer and the BOX layer. Therefore, the quality of the low-dose SIMOX wafers can be significantly improved. However, even in the low-dose SIMOX process in which this technology is adopted, due to the large dose amount of oxygen ions, the ion-implantation takes several hours per batch. Moreover, since the ITOX process (i.e., specific heat treatment steps) is needed, productivity is decreased. Therefore, there is a problem to increase production costs.
With regard to the method for manufacturing SIMOX wafers, a process in which oxygen ions are implanted in two separate stages has been disclosed (Patent Document 2). In this two-stage oxygen ion implantation, oxygen ions are implanted into a silicon wafer at a large dose amount while the silicon wafer is in a heated state, and then the silicon wafer is cooled to about room temperature and oxygen ions are again implanted. In the first oxygen ion implantation, since the silicon wafer is heated, the surface of the silicon wafer is kept in the form of a silicon single crystal. In the second oxygen ion implantation, since the silicon wafer is held at a low temperature, an amorphous layer is formed. Then, the silicon wafer thus obtained is subjected to an oxidation treatment at a temperature such as 1350° C. for a fixed length of time, thereby, a SOI structure is formed.
In this process, due to the heat treatment after the ion implantation, a high-density defect layer including polysilicon, twin crystal and stacking faults are formed from the amorphous layer. Since oxygen readily precipitates in a region in which this high-density defect layer is formed, it is possible to thicken the BOX layer up to about twice as thick as a theoretical film thickness anticipated from the dose amount of oxygen ions. Moreover, since the dose amount of oxygen ions can be reduced to a lower level than that in the ITOX process, productivity is improved and production costs can be reduced. SIMOX wafers manufactured by this process are referred to as MLD-SIMOX, which stands for “modified low dose SIMOX”.
Meanwhile, in the above steps or some other manufacturing steps, defects may be formed in the surface of the silicon substrate and particles such as dust or the like may adhere to the surface of the SOI wafer. If the SOI wafer is used in subsequent steps while remaining in such an undesirable state, there is a risk of decreasing the production yield of devices fabricated from the SOI wafer. Hence, the SOI wafer is subjected to an inspection to detect particles. For example, the surface of the substrate is cleaned and dried, and then, the inspection of the SOI wafer is typically carried out by exposing a light onto the surface of the SOI wafer and using an apparatus such as a surface inspection system to detect particles.
In the above MLD-SIMOX wafer, when a BOX layer is formed, the roughness increases at the surface of the SOI layer and at the interface between the SOI layer and the BOX layer. This may make it impossible, in the above particle inspection, to distinguish between the roughness of the surface of the SOI layer and the particles.
Another problem is that, in the MLD-SIMOX wafer, when the dose amount of oxygen ions is increased in order to achieve a thicker BOX layer, the breakdown voltage of the BOX layer is lowered.
It is therefore an object of the present invention to suppress the increases in roughness at the surface of the SOI layer and at the interface between the SOI layer and the BOX layer and the decrease in breakdown voltage of the BOX layer that are associated with the thickening of the BOX layer in the MLD-SIMOX wafer.
(Patent Document 1) Japanese Patent Application, First Publication No. H07-263538
(Patent Document 2) U.S. Pat. No. 5,930,643
(Non-Patent Document 1) S. Nakashima, et al., “Thickness Increment of Buried Oxide in a SIMOX Wafer by High-Temperature Oxidation,” Proceedings of 1994 IEEE International SOI Conference, p. 71 to 72 (1994)